/**
 * Copyright (C) 2015-2017 Lightbend Inc. <http://www.lightbend.com>
 */
package akka.pattern

import scala.concurrent.duration.{ Duration, FiniteDuration }
import akka.actor.{ Props, OneForOneStrategy, SupervisorStrategy }

/**
 * Builds back-off options for creating a back-off supervisor.
 * You can pass `BackoffOptions` to `akka.pattern.BackoffSupervisor.props`.
 * An example of creating back-off options:
 * {{{
 * Backoff.onFailure(childProps, childName, minBackoff, maxBackoff, randomFactor)
 *               .withManualReset
 *               .withSupervisorStrategy(
 *                 OneforOneStrategy(){
 *                    case e: GivingUpException => Stop
 *                    case e: RetryableException => Restart
 *                 }
 *               )
 *               .withReplyWhileStopped(TheSystemIsDown)
 *
 * }}}
 */
object Backoff {
  /**
   * Back-off options for creating a back-off supervisor actor that expects a child actor to restart on failure.
   *
   * This explicit supervisor behaves similarly to the normal implicit supervision where
   * if an actor throws an exception, the decider on the supervisor will decide when to
   * `Stop`, `Restart`, `Escalate`, `Resume` the child actor.
   *
   * When the `Restart` directive is specified, the supervisor will delay the restart
   * using an exponential back off strategy (bounded by minBackoff and maxBackoff).
   *
   * This supervisor is intended to be transparent to both the child actor and external actors.
   * Where external actors can send messages to the supervisor as if it was the child and the
   * messages will be forwarded. And when the child is `Terminated`, the supervisor is also
   * `Terminated`.
   * Transparent to the child means that the child does not have to be aware that it is being
   * supervised specifically by this actor. Just like it does
   * not need to know when it is being supervised by the usual implicit supervisors.
   * The only caveat is that the `ActorRef` of the child is not stable, so any user storing the
   * `sender()` `ActorRef` from the child response may eventually not be able to communicate with
   * the stored `ActorRef`. In general all messages to the child should be directed through this actor.
   *
   * An example of where this supervisor might be used is when you may have an actor that is
   * responsible for continuously polling on a server for some resource that sometimes may be down.
   * Instead of hammering the server continuously when the resource is unavailable, the actor will
   * be restarted with an exponentially increasing back off until the resource is available again.
   *
   * '''***
   * This supervisor should not be used with `Akka Persistence` child actors.
   * `Akka Persistence` actors shutdown unconditionally on `persistFailure()`s rather
   * than throw an exception on a failure like normal actors.
   * [[#onStop]] should be used instead for cases where the child actor
   * terminates itself as a failure signal instead of the normal behavior of throwing an exception.
   * ***'''
   * You can define another
   * supervision strategy by using `akka.pattern.BackoffOptions.withSupervisorStrategy` on [[akka.pattern.BackoffOptions]].
   *
   * @param childProps the [[akka.actor.Props]] of the child actor that
   *   will be started and supervised
   * @param childName name of the child actor
   * @param minBackoff minimum (initial) duration until the child actor will
   *   started again, if it is terminated
   * @param maxBackoff the exponential back-off is capped to this duration
   * @param randomFactor after calculation of the exponential back-off an additional
   *   random delay based on this factor is added, e.g. `0.2` adds up to `20%` delay.
   *   In order to skip this additional delay pass in `0`.
   */
  def onFailure(
    childProps:   Props,
    childName:    String,
    minBackoff:   FiniteDuration,
    maxBackoff:   FiniteDuration,
    randomFactor: Double): BackoffOptions =
    BackoffOptionsImpl(RestartImpliesFailure, childProps, childName, minBackoff, maxBackoff, randomFactor)

  /**
   * Back-off options for creating a back-off supervisor actor that expects a child actor to stop on failure.
   *
   * This actor can be used to supervise a child actor and start it again
   * after a back-off duration if the child actor is stopped.
   *
   * This is useful in situations where the re-start of the child actor should be
   * delayed e.g. in order to give an external resource time to recover before the
   * child actor tries contacting it again (after being restarted).
   *
   * Specifically this pattern is useful for persistent actors,
   * which are stopped in case of persistence failures.
   * Just restarting them immediately would probably fail again (since the data
   * store is probably unavailable). It is better to try again after a delay.
   *
   * It supports exponential back-off between the given `minBackoff` and
   * `maxBackoff` durations. For example, if `minBackoff` is 3 seconds and
   * `maxBackoff` 30 seconds the start attempts will be delayed with
   * 3, 6, 12, 24, 30, 30 seconds. The exponential back-off counter is reset
   * if the actor is not terminated within the `minBackoff` duration.
   *
   * In addition to the calculated exponential back-off an additional
   * random delay based the given `randomFactor` is added, e.g. 0.2 adds up to 20%
   * delay. The reason for adding a random delay is to avoid that all failing
   * actors hit the backend resource at the same time.
   *
   * You can retrieve the current child `ActorRef` by sending `BackoffSupervisor.GetCurrentChild`
   * message to this actor and it will reply with [[akka.pattern.BackoffSupervisor.CurrentChild]]
   * containing the `ActorRef` of the current child, if any.
   *
   * The `BackoffSupervisor`delegates all messages from the child to the parent of the
   * `BackoffSupervisor`, with the supervisor as sender.
   *
   * The `BackoffSupervisor` forwards all other messages to the child, if it is currently running.
   *
   * The child can stop itself and send a [[akka.actor.PoisonPill]] to the parent supervisor
   * if it wants to do an intentional stop.
   *
   * Exceptions in the child are handled with the default supervisionStrategy, which can be changed by using
   * [[BackoffOptions#withSupervisorStrategy]] or [[BackoffOptions#withDefaultStoppingStrategy]]. A
   * `Restart` will perform a normal immediate restart of the child. A `Stop` will
   * stop the child, but it will be started again after the back-off duration.
   *
   * @param childProps the [[akka.actor.Props]] of the child actor that
   *   will be started and supervised
   * @param childName name of the child actor
   * @param minBackoff minimum (initial) duration until the child actor will
   *   started again, if it is terminated
   * @param maxBackoff the exponential back-off is capped to this duration
   * @param randomFactor after calculation of the exponential back-off an additional
   *   random delay based on this factor is added, e.g. `0.2` adds up to `20%` delay.
   *   In order to skip this additional delay pass in `0`.
   */
  def onStop(
    childProps:   Props,
    childName:    String,
    minBackoff:   FiniteDuration,
    maxBackoff:   FiniteDuration,
    randomFactor: Double): BackoffOptions =
    BackoffOptionsImpl(StopImpliesFailure, childProps, childName, minBackoff, maxBackoff, randomFactor)
}

/**
 * Configures a back-off supervisor actor. Start with `Backoff.onStop` or `Backoff.onFailure`.
 * BackoffOptions is immutable, so be sure to chain methods like:
 * {{{
 * val options = Backoff.onFailure(childProps, childName, minBackoff, maxBackoff, randomFactor)
 *               .withManualReset
 * context.actorOf(BackoffSupervisor.props(options), name)
 * }}}
 */
trait BackoffOptions {
  /**
   * Returns a new BackoffOptions with automatic back-off reset.
   * The back-off algorithm is reset if the child does not crash within the specified `resetBackoff`.
   * @param resetBackoff The back-off is reset if the child does not crash within this duration.
   */
  def withAutoReset(resetBackoff: FiniteDuration): BackoffOptions

  /**
   * Returns a new BackoffOptions with manual back-off reset. The back-off is only reset
   * if the child sends a `BackoffSupervisor.Reset` to its parent (the backoff-supervisor actor).
   */
  def withManualReset: BackoffOptions

  /**
   * Returns a new BackoffOptions with the supervisorStrategy.
   * @param supervisorStrategy the supervisorStrategy that the back-off supervisor will use.
   *   The default supervisor strategy is used as fallback if the specified supervisorStrategy (its decider)
   *   does not explicitly handle an exception. As the BackoffSupervisor creates a separate actor to handle the
   *   backoff process, only a [[OneForOneStrategy]] makes sense here.
   */
  def withSupervisorStrategy(supervisorStrategy: OneForOneStrategy): BackoffOptions

  /**
   * Returns a new BackoffOptions with a default `SupervisorStrategy.stoppingStrategy`.
   * The default supervisor strategy is used as fallback for throwables not handled by `SupervisorStrategy.stoppingStrategy`.
   */
  def withDefaultStoppingStrategy: BackoffOptions

  /**
   * Returns a new BackoffOptions with a constant reply to messages that the supervisor receives while its
   * child is stopped. By default, a message received while the child is stopped is forwarded to `deadLetters`.
   * With this option, the supervisor will reply to the sender instead.
   * @param replyWhileStopped The message that the supervisor will send in response to all messages while
   *   its child is stopped.
   */
  def withReplyWhileStopped(replyWhileStopped: Any): BackoffOptions

  /**
   * Returns the props to create the back-off supervisor.
   */
  private[akka] def props: Props
}

private final case class BackoffOptionsImpl(
  backoffType:        BackoffType          = RestartImpliesFailure,
  childProps:         Props,
  childName:          String,
  minBackoff:         FiniteDuration,
  maxBackoff:         FiniteDuration,
  randomFactor:       Double,
  reset:              Option[BackoffReset] = None,
  supervisorStrategy: OneForOneStrategy    = OneForOneStrategy()(SupervisorStrategy.defaultStrategy.decider),
  replyWhileStopped:  Option[Any]          = None) extends BackoffOptions {

  val backoffReset = reset.getOrElse(AutoReset(minBackoff))

  def withAutoReset(resetBackoff: FiniteDuration) = copy(reset = Some(AutoReset(resetBackoff)))
  def withManualReset = copy(reset = Some(ManualReset))
  def withSupervisorStrategy(supervisorStrategy: OneForOneStrategy) = copy(supervisorStrategy = supervisorStrategy)
  def withDefaultStoppingStrategy = copy(supervisorStrategy = OneForOneStrategy()(SupervisorStrategy.stoppingStrategy.decider))
  def withReplyWhileStopped(replyWhileStopped: Any) = copy(replyWhileStopped = Some(replyWhileStopped))

  def props = {
    require(minBackoff > Duration.Zero, "minBackoff must be > 0")
    require(maxBackoff >= minBackoff, "maxBackoff must be >= minBackoff")
    require(0.0 <= randomFactor && randomFactor <= 1.0, "randomFactor must be between 0.0 and 1.0")
    backoffReset match {
      case AutoReset(resetBackoff) ⇒
        require(minBackoff <= resetBackoff && resetBackoff <= maxBackoff)
      case _ ⇒ // ignore
    }

    backoffType match {
      //onFailure method in companion object
      case RestartImpliesFailure ⇒
        Props(new BackoffOnRestartSupervisor(childProps, childName, minBackoff, maxBackoff, backoffReset, randomFactor, supervisorStrategy, replyWhileStopped))
      //onStop method in companion object
      case StopImpliesFailure ⇒
        Props(new BackoffSupervisor(childProps, childName, minBackoff, maxBackoff, backoffReset, randomFactor, supervisorStrategy, replyWhileStopped))
    }
  }
}

private sealed trait BackoffType
private final case object StopImpliesFailure extends BackoffType
private final case object RestartImpliesFailure extends BackoffType

private[akka] sealed trait BackoffReset
private[akka] final case object ManualReset extends BackoffReset
private[akka] final case class AutoReset(resetBackoff: FiniteDuration) extends BackoffReset
